"Stretch" is a description of wear -- as the pins and links of the chain wear, it gets longer, and since it still has the same number of links, the angular relationship between the gears changes -- the chain path is the same but the links are further apart. This makes the cam timing late, and eventually the chain tensioner cannot keep the chain tight -- deadly in these cars, as it will "slap" and break the chain guide on the right head, get the broken part fouled in the slack, and bend the valves on the right head. Costs $5000-$7000 to fix.

The amount of stretch, measured in degrees of rotation "lag",depends upon where you measure it, cam or crank -- the cam goes half the distance of the crank per revolution, so 4 on the cam is 8 on the crank - the limit my indy mechanic uses.

The double roller chain is a retofit, and vastly preferable to the original single chain (unless yours is a late version, when the factory did the change).

Getting the chain replaced is vastly perferable to letting it get long enought to break!